1. Field of the Invention
This invention concerns mesoporous macrostructures composed of a microporous inorganic material which macrostructures can have controlled size, shape, and/or porosity and a process for production of such macrostructures.
2. Description of the Related Art
Both mesoporous inorganic material and microporous inorganic material are characterized by a large specific surface area in pores and are used in a large number of applications of considerable commercial importance. The terms “porous inorganic material” and “porous material”, as used herein, includes mesoporous inorganic material, microporous inorganic material, and mixtures or combinations thereof.
In most applications which use porous inorganic material, the fact that the phase interface between the solid porous material and the medium (liquid or gas) in which such use occurs is large can be very important. Heterogeneous phase catalysts used in refinery processes, petrochemical conversion processes, and different environmentally related applications often comprise a use of porous inorganic material, especially microporous material. Adsorbents for the selective adsorption in the gas or liquid phase or the selective separation of ionic compounds are often porous inorganic material. In addition to these applications, porous inorganic materials have recently become increasingly utilized in a number of more technologically advanced areas. Examples of such uses include use in chemical sensors, in fuel cells and batteries, in membranes for separation or catalytic purposes, during chromatography for preparative or analytical purposes, in electronics and optics, and in the production of different types of composites.
Although a large phase interface is often a fundamental requirement for use of porous materials in different applications, a number of additional requirements related to the specific area of application are imposed on these materials. For example, the large phase interface available in the pores of the porous inorganic material must be accessible and useable. Therefore, the porosity, pore size and pore size distribution in large pores (meso- and macropores) are often of major significance, especially when mass transport affects process performance. The surface properties of the porous material can also be very important for the performance of the material in a given application. In this context, the purity of the material comprising the macrostructure is also significant.
In most applications, size and shape of porous macrostructures containing the porous inorganic material and the degree of variation of these properties are very important. During use, the size and shape of the porous macrostructures can influence properties like mass transport within the porous structures, pressure drop over a bed of particles of the macrostructure material, and the mechanical and thermal strength of the macrostructure material. The factors that are the most important will vary, depending on the application in which the macrostructures are used, as well as the configuration of the process in which the application occurs. Techniques that permit production of macrostructure materials with increased specific surface area, pore structure (pore size/pore size distribution), chemical composition, mechanical and thermal strength, as well as increased and uniform size and shape, are consequently required to tailor porous inorganic macrostructures to different applications.
Mesoporous inorganic materials include amorphous metal oxide (non-crystalline) materials which have mesoporous and, optionally, partially microporous structure. The pore size of the mesoporous inorganic material is usually in the range of from about 20 Å to about 500 Å.
Microporous inorganic materials include crystalline molecular sieves. Molecular sieves are characterized by the fact that they are microporous materials with pores of a well-defined size ranging discreetly from about 2 Å to about 20 Å. Most molecules, whether in the gas or liquid phase, both inorganic and organic, have dimensions that fall within this range at room temperature. Selecting a molecular sieve composition with a suitable and discreet pore size therefore allows separation of specific molecules from a mixture with other molecules of a different size through selective adsorption, hence the name “molecular sieve”. Apart from the selective adsorption and selective separation of uncharged molecular sieve particles, the well-defined and discreet pore system of a molecular sieve enables selective ion exchange of charged particles and selective catalysis. In the latter two cases, significant properties other than the micropore structure include, for instance, ion exchange capacity, specific surface area and acidity.
Molecular sieves can be classified into various categories such as by their chemical composition and their structural properties. A group of molecular sieves of commercial interest is the group comprising the zeolites, which are defined as crystalline aluminosilicates. Another group is that of the metal silicates, structurally analogous to zeolites, but for the fact that they are substantially free of aluminum (or contain only very small amounts thereof). Still another group of molecular sieves are ALPO-based molecular sieves which contain framework tetrahedral units of alumina (AlO2) and phosphorous oxide (PO2) and, optionally, silica (SiO2). Examples of such molecular sieves include SAPO, ALPO, MeAPO, MeAPSO, ELAPO, and ELAPSO.
A summary of the prior art, in terms of production, modification and characterization of molecular sieves, is described in the book “Molecular Sieves—Principles of Synthesis and Identification”; (R. Szostak, Blackie Academic & Professional, London, 1998, Second Edition). In addition to molecular sieves, amorphous materials, chiefly silica, aluminum silicate and aluminum oxide, have been used as adsorbents and catalyst supports. A number of long-known techniques, like spray drying, prilling, pelletizing and extrusion, have been and are being used to produce macrostructures in the form of, for example, spherical particles, extrudates, pellets and tablets of both micropores and other types of porous materials for use in catalysis, adsorption and ion exchange. A summary of these techniques is described in “Catalyst Manufacture,” A. B. Stiles and T. A. Koch, Marcel Dekker, New York, 1995.
Because of limited possibilities with the known techniques to produce macrostructures of amorphous and/or molecular sieve type compositions, considerable investment has been made to find new ways to produce macrostructures of such porous inorganic materials, with a certain emphasis on those in the form of films, with a controllable pore structure together with a requisite mechanical strength of the macrostructure.
PCT Publication WO 94/25151 involves the production of films of molecular sieves by a process in which seed crystals of molecular sieves are deposited on a substrate surface and then made to grow together into a continuous film. PCT Publication WO 94/25152 involves the production of films of molecular sieves by introduction of a substrate to a synthesis solution adjusted for zeolite crystallization and crystallization with a gradual increase in synthesis temperature. PCT Publication WO 94/05597 involves the production of colloidal suspensions of identical microparticles of molecular sieves with an average size below 200 nm. PCT Publication WO 90/09235 involves a method for production of an adsorbent material in the form of a monolith by impregnation of a monolithic cell structure with a hydrophobic molecular sieve, followed by partial sintering of the molecular sieve with the material from which the cell structure is constructed.
Although a number of different techniques already exist for production of porous inorganic macrostructures with the desired size and shape, these techniques have a number of limitations that can affect the properties and performance of the macrostructures during their use. Most of these techniques require the use of an amorphous binder to give the macrostructure acceptable mechanical strength. The presence of the such amorphous binder can adversely affect certain desired properties, such as high specific surface area and uniform chemical composition. Also, most of the existing binding techniques constrain the ability to tailor the macrostructure, in size and shape, within narrow limits. If a well defined size is desired with a narrow particle size distribution, it is many times necessary, and most often required, to separate desirable and undesirable macrostructures, which can lead to considerable waste during manufacture.
The use of different types of binders can also affect the pore structure of the macrostructures and it is often necessary to find a compromise between mechanical properties and pore size. Often it is desirable to have a bimodal pore size distribution in the macrostructures of the porous materials, in which the micropores maintain a large specific phase interface, whereas the larger pores existing in the macrostructure in the meso- or macropore range permit transport of molecules to these microporous surfaces and, in this way, prevent diffusion limitations. During production of macrostructures using the heretofore known techniques, a secondary system of pores within the meso- and/or macropore range can be produced by admixing a particulate inorganic material with an organic material (for example, cellulose fibers), which is later eliminated by calcining. These techniques, however, often produce an adverse effect on the other properties of the resulting macrostructure material.